The visual cortex is widely held to represent stimulus attributes through the activity of neuronal populations. Goal of this research is to measure these population responses, and in particular to establish how they develop in time (their dynamics). Population responses will be recorded in primary visual cortex (V1) through imaging of voltage-sensitive dyes (VSD). VSD imaging delivers excellent resolution in space and time. These measurements will test the predictions of three basic architectures that have been hypothesized for the circuitry of V1: feedforward, feedback-sharpening, and feedback-broadening. These architectures could coexist and serve different roles, e.g. one could underlie selectivity for stimulus position and another could underlie selectivity for stimulus position. Aim 1: Dynamics of stimulus selectivity. The three architectures make distinct predictions for how the selectivity of population activity evolves during a response. These predictions will be tested through VSD imaging. Aim 2: Functional connectivity. The three architectures involve different layouts of excitatory and inhibitory connections. These layouts will be probed through cross-correlation of VSD signals, extending to populations a method commonly used with single neurons. Aim 3: Adaptation of stimulus selectivity. The three architectures make distinct predictions regarding the effects of visual adaptation. These predictions will be tested through VSD imaging of population responses. Aim 4: Dynamics of ongoing activity. A feedback architecture with strong intracortical connections creates quasi-stable patterns of activity (attractors). The existence of these attractors will be tested by VSD imaging responses to a variety of stimuli, including stimuli whose orientation is ambiguous. Understanding population responses in visual cortex is a key step to advance our knowledge of how the visual system functions in health and disease, a goal at the core of the mission of the National Eye Institute. Relevance: Understanding the working of primary visual cortex will benefit the understanding of the whole cortex, and therefore help develop treatments for pathologies that stem from cortical dysfunction, including epilepsy and migraine. Indeed, the cerebral cortex is widely held to be organized according to a limited number of functional principles, and these principles are best studied in an area that we understand in depth.